Choline esters of aromatic or higher molecular weight acids serve as what?

• A. Cholinergic agonists; active at receptor
• B. Cholinergic antagonists; too bulky to activate the receptor
• C. Cholinergic partial agonists
• D. No effect on receptor binding

Answer: (B)

The important idea is how the size and shape of a cholinergic ligand determine whether it can activate the receptor. To turn on the receptor, the ligand must fit the binding site precisely and orient its functional groups to induce the conformational change that starts signaling. Choline esters formed with aromatic or other large acid moieties are too bulky to trigger that activation, even though they can still fit into the binding pocket. They effectively occupy the receptor and block acetylcholine or other agonists from activating it, acting as competitive antagonists.